Xcode 11.6 beta is now available! 
To select this Stack you just have to open your app on bitrise.io,
go to the Workflow tab (Workflow Editor), and on the Stack tab select the Xcode 11.6.x
option and your next build will start on the Xcode 11.6 stack.
Note: you can now select separate stacks for separate workflows!
Read more at: https://blog.bitrise.io/per-workflow-stack-selection-is-here.
You can find the system report here:
https://github.com/bitrise-io/bitrise.io/commit/66a43c390a8dc93183ee8bb941ca35127e931bc5
If you’d like to add additional tools to be pre-installed you can find the instructions on GitHub, for both the Linux and for the macOS stacks.
Information about Stack types & update schedules can be found here: discuss.bitrise.io/t/281.
Happy Building! 